The present invention relates to a method of extruding a mixture made of a first and a second thermoplastic.
A method of producing conducting wires coated with cross-linked polyethylene is known from European Patent Application 1 148 518, in which a mixture of granulate, chips, or powder made of a polyethylene homopolymer and a polyethylene copolymer is coated using a liquid cross-linking agent, the coated granulate, chips, or powder is melted in an extruder and extruded onto the conducting wire, and the extruded coating is cross-linked by heating to a temperature above the decomposition temperature of the cross-linking agent.
A method of extruding thermoplastically workable polymer plastics is known from British Patent 2 166 079. The function of an extruder is to melt and homogenize the thermoplastic material and to supply it under pressure at a uniform rate to an extrusion nozzle, using which the thermoplastic melt is brought into the desired shape. In the extrusion method, powder, granulate, or pellets of the thermoplastic are fed into the intake region of an extruder screw, which rotates inside a heated extruder housing. The thermoplastic material is transported by the rotation of the extruder screw inside the extruder housing in the direction toward the extrusion nozzle and is melted and homogenized at the same time. The melting and homogenization are performed in this case by supplying heat, which arises through the shear forces and is also introduced into the inside of the extruder by heating the extruder housing from the outside.
The delivery of an extruder is a function of, among other things, the geometric dimensions of the extruder screw and extruder housing, the rotation speed of the extruder screw, the properties of the thermoplastic, the geometric dimensions of the delivery zone and the extrusion nozzle, and the pressure applied in the region of the delivery zone.
In order to elevate the performance of an extruder having an extruder screw, it is suggested in British Patent 2 166 079 that the extruder screw be provided with a hole and an electrical heating element be inserted into the hole in order to heat the melt region of the extruder screw from the inside.
Problems arise during the extrusion of a mixture of two thermoplastics whose melting ranges are at different levels in that the granulate, powder, or pellets adhere to the surface of the screw in the intake region of the extruder screw when it is heated, and stable conveyance of the granulate, the powder, and the pellets is therefore no longer ensured.
In addition, the screw is heated in the region of the delivery zone by the heating from the outside, but particularly by the shear forces, to a temperature at which any additives which are admixed in order to improve the properties of the plastic already begin to react, which may be undesirable under certain circumstances.